1. Field of the Invention
This invention relates generally to MOS switches, and more particularly, this invention relates to a technique to improve the linearity of a MOS switch beyond that achievable using known methods and structures.
2. Description of the Prior Art
MOS switches are commonly employed in both continuous time applications and sampling applications. Programmable gain amplifiers, for example, include MOS switches to select input and feedback resistors to achieve a desired gain. A typical sample and hold (S/H) circuit in a CMOS process employs two basic devices, a MOS switch and capacitor. The switch works in its triode region as a MOS resistor. Harmonic distortions (HDs) are generated when AC current charging the sampling capacitor Cs passes through the resistor whenever the capacitor and/or the resistor are nonlinear. As used herein, HD3 means third harmonic distortion.
A S/H circuit 10 can be modeled as shown in FIG. 1 with an NMOS switch 12 and Node B that is connected to VSS. Sample and hold circuit 10 has a linear source resistance Rs 14, two nonlinear parasitic junction capacitances Cj 16, 18 and a sampling capacitance Cs 20. Considering only the tracking accuracy, with a conventional connection (i.e. Node B connected to VSS), the differential equation governing the tracking operation is                               C          s                ⁢                              ⅆ                          V              out                                            ⅆ            t                              +                                    ⅆ                          C              j                                ⁢                      V            out                                    ⅆ          t                      =          K      ⁡              [                                            (                                                V                  G0                                -                                                      (                                          1                      -                      β                                        )                                    ⁢                                      V                    s                                                  -                                  V                  T                                            )                        ⁢                          V              ds                                -                                    1              2                        ⁢                          V              2                        ⁢                          ⅆ                              s                2                                                    ]              ,where β determines the signal content in the gate voltage VG, and Cj 16, 18 are the junction capacitances of source and drain. Without solving the equation, it can be observed that current/voltage (I-V) curve is nonlinear, which results in a nonlinear channel resistance resistor. The problem with a conventional sampling switch then is that the switch on-resistance is input signal dependent, and generates harmonic distortion.
Programmable gain amplifiers such as illustrated in FIG. 8 that shows an inverting amplifier 200 and FIG. 9 that shows a differencing amplifier 300 are often also made with MOS switches in which MOS or CMOS transfer gates (switches) are used to select input and feedback resistors to achieve a desired gain. As discussed herein before, distortion occurs due to the modulation of the gate-to-source voltage of the active switches caused by the switch source/drain node voltage varying with the input or feedback signal. (It should be noted that in the case of MOS devices used for bi-directional transfer gates, either of the two source/drain terminals can be labeled as source or drain depending on the direction of instantaneous signal flow).
To reduce this distortion, the switches shown in FIGS. 8 and 9 are generally placed with one end attached to an opamp input which does not vary in voltage as much as other circuit nodes. This is especially true of the inverting amplifier configuration shown in FIG. 8 in which the negative input of opamp 202 acts as a virtual ground node. For the differencing amplifier configuration shown in FIG. 9, however, the voltages associated with both opamp 302 inputs vary significantly with the signal. Some improvement in switch linearity is obtained by scaling the switches so that their effective resistances are small compared to the resistors with which they are in series.
Another known method of reducing distortion is to generate signals that are level-shifted replicas of the input signal and use them to drive the gates of the NMOS and PMOS devices in an active switch so that their gate-to-source voltages are approximately constant for all values of the input signal. FIG. 2 illustrates a technique proposed by others in which the switch gate voltage VG is bootstrapped to be signal dependent (with β=1) and VGS is made constant. The signal dependent VGS, that introduces a nonlinear switch on-resistance which is considered to be a first order effect, is then removed. This known technique is still problematic however, in that a residual non-linearity remains due to the modulation of the source to bulk (back gate) voltage.
In view of the foregoing, it is both advantageous and desirable to provide a technique to further improve the linearity of a MOS switch beyond that achievable using known methods and structures.